1. Field of the Invention
The present invention relates to a receiver suitable for data transmission, which transmits data through the use of a satellite broadcasting system.
2. Description of the Related Art
A conventional receiver for data transmission (hereinafter called simply "a receiver") will be explained in accordance with FIG. 2. FIG. 2 shows a part of a circuit for the conventional receiver.
A data transmission signal is first transmitted from a satellite in a 11 GHz to 12 GHz band corresponding to the same frequency band as that for satellite broadcasting signals. The data transmission signal is frequency-converted to a band of about 950 MHz to 2150 MHz by an unillustrated down converter provided outdoors, after which it is inputted to an input terminal 31 of the receiver.
The data transmission signal (hereinafter called "received signal") inputted to the input terminal 31 is amplified by a low-noise amplifier 32 after which it is inputted to a mixer 34 through a first variable attenuator 33. An unillustrated PIN diode is used for the first variable attenuator 33. The amount of attenuation thereof is controlled by a current which flows through the PIN diode. The mixer 34 is comprised of a dual gate MOS FET (Field Effect Transistor) (hereinafter called simply "FET"). The received signal is inputted to a first gate of the FET. Each local oscillation signal generated from a local oscillator 35 is inputted to a second gate of the FET through a buffer amplifier 36. The mixer 34 frequency-converts the received signal to an intermediate frequency signal whose center frequency is about 480 MHz. Thus, the local oscillator 35 generates oscillations between about 1430 MHz and 2630 MHz in association with a frequency band for the received signal.
The intermediate frequency signal is outputted from the drain of the FET for the mixer 34 and inputted to a second variable attenuator 38 through an impedance matching circuit 37. The impedance matching circuit 37 is used to match the output impedance of the FET corresponding to the mixer 34 to the impedance of the second variable attenuator 38 and comprises a capacitor 37a, an inductor 37b and a resistor 37c electrically connected in parallel. A PIN diode is used even for the second variable attenuator 38 although not shown in the drawing. The amount of attenuation thereof is controlled by a current which flows through the PIN diode.
The intermediate frequency signal whose level is controlled by the second variable attenuator 38, is inputted to a demodulator 42 via a first intermediate frequency amplifier 39, an intermediate frequency filter 40 and a second intermediate frequency amplifier 41 in that order. The intermediate frequency filter 40 is made up of a bandpass filter whose center frequency is about 480 MHz and whose bandwidth is about 30 MHz. Further, the intermediate frequency signal is demodulated by the demodulator 42 from which baseband signals (I signal and Q signal) are outputted.
On the other hand, the output of the second intermediate frequency amplifier 41 is inputted even to a detector 43. The detector 43 detects it and outputs an AGC voltage therefrom. The output AGC voltage is suitably amplified by a DC amplifier 44, followed by supply to the first variable attenuator 33 and the second variable attenuator 38, whereby the currents flowing through their PIN diodes are controlled.
That is, the amount of attenuation of the first variable attenuator 33 and the amount of attenuation of the second variable attenuator 38 are controlled according to the AGC voltage obtained based on the output of the second intermediate frequency amplifier 41, whereby the level of the received signal corresponding to the RF signal inputted to the mixer 34 and the level of the intermediate frequency signal inputted to the demodulator 42 are controlled so as to become approximately constant respectively.
In the conventional receiver described above, the first variable attenuator 33 and the second variable attenuator 38 are provided on the input and output sides of the mixer 34 respectively to control the level of the RF signal inputted to the mixer 34 and the level of the intermediate frequency signal inputted to the demodulator 42 so that they are held constant. Since, however, the PIN diodes are respectively used for the first variable attenuator 33 and the second variable attenuator 38, they are excited by the RF signal and the intermediate frequency signal respectively. As a result, large white noise is produced because the PIN diode used for the second variable attenuator 38 is particularly excited by the intermediate frequency signal whose level is made high by conversion gain of the mixer 34. In doing so, the white noise enters into the local oscillator 35 side through terminal-to-terminal capacitance between the second gate and drain of the FET constituting the mixer 34, so that a C/N ratio between the local oscillation signals is deteriorated.
That is, since a terminal-to-terminal capacitance of about 0.05 pF exists between the drain and second gate of the FET and a series resonance circuit having a resonance frequency of about 1800 MHz is formed between the terminal-to-terminal capacitance and the inductor 37b constituting the impedance matching circuit 37, a frequency component close to the resonance frequency, of the white noise produced from the PIN diode for the second variable attenuator 38 enters the local oscillator 35 to deteriorate the C/N ratio between the local oscillation signals.
Thus, since the received signal inputted to the mixer 34 is frequency-converted to the intermediate frequency signal, based on the local oscillation signals poor in C/N ratio, phase noise is superimposed on the frequency-converted intermediate frequency signal. Therefore, a problem arises in that so-called bit errors occur in data extracted based on I and Q signals corresponding to the baseband signals outputted from the demodulator 42. Its bit-error rate tends to increase as the amount of transmission of data decreases. This is because the ratio of noise to the number of data is considered to increase.